Flow and Changes in Appearance

Julie Dorsey, Hans Pedersen, and Pat Hanrahan

Submitted to SIGGRAPH '96

Abstract

An important, largely unexplored area of computer image generation is the effect of weathering on appearance. Weathering results from the interaction of the environment with the materials in the world. Of the natural forces involved in the weathering of materials, the flow of water plays a major role---producing a distinctive set of patterns of washes and stains. This paper presents an intuitive phenomenological model for the flow of water over surfaces that is capable of generating such changes in appearance.

We represent the flow with a particle system and differential equations. These coupled differential equations control both the rate of absorption of water by the surface, and the rate of sedimentation of deposits on the surface. The motion of the water particles is governed by forces due to gravity, friction, wind, roughness and constraints that maintain contact with the surface. The surface material model contains parameters such as absorption, solubility and adhesion that govern the interaction of water and deposits with the surface.

The system runs in both batch and interactive modes, allowing the user to direct the outcome of the simulation. A wide range of effects can be simulated by simply adjusting parameters affecting the flow until the desired effects are achieved. To illustrate the power of this simple model, we show two examples of flows over complex geometries and the resulting changes in appearance: a building facade modeled as polygons, and a statue made from bicubic patches. The resulting patterns are striking, and very difficult to achieve using traditional texturing techniques.

Additional information

Compressed PostScript of full paper without color plates (337KB)(6MB uncompressed).

Plate 1 (2MB).

Plate 2 (2MB)

Plate 3 (2MB)

Plate 4 (2MB)

Plate 5 (2MB)

Last update: 20 January 1996

dorsey@lcs.mit.edu